Long-term tillage effects on nitrate movement and accumulation and denitrification in the root and intermediate vadose zones

This study evaluated long-term (15 years) tillage effects on denitrification and nitrate accumulation and movement in the root and intermediate vadose zones (IVZ). Bromide was used as an analogy to nitrate when studying the nitrate movement. Effects of crop rotation on nitrate leaching and the occurrence of atrazine in IVZ were also investigated. The tillage treatments were disk-and-surface plant (DSP), chisel-and-surface plant (CSP), ridge-till (RT), and slot-plant (SP). The crop rotations were continuous corn (CC) and corn/soybean (CS). The root-zone nitrate level was greater in 7 out of 8 years with DSP than RT and SP. Despite the uniform N rate, yield was higher with RT and SP than DSP. With RT and SP, more soil-N was removed in grain than with DSP. The rates of nitrate movement in IVZ of all tillage systems ranged from 0.87-0.92 and 0.73-0.78 m yr$\sp{-1}$ at upstream and downstream ends of furrows. Nitrate accumulation in the IVZ of DSP was 2.1-2.4 and 1.6-2.2 times greater than that of RT and SP. Over 15 years, more nitrate leached from the root zone of DSP compared to RT and SP. Some Br$\sp-$ was bypassed by water with both tillage systems. There was no difference in bypass flow between tillage systems. When the surface residue was removed, the water infiltrated and Br$\sp-$ leached were not different between CSP and SP. When the surface residue was not removed the water infiltrated and Br$\sp-$ leached were significantly greater with SP than DSP. The crop residue on the soil surface retained some Br$\sp-$ and retarded leaching. Denitrification potential at the soil surface varied from 899 to 14 g-N ha$\sp{-1}$ day$\sp{-1}$ from April through July, respectively. Denitrification was not significantly different among DSP, RT, and SP tillage at any time. There was no evidence of denitrification in the IVZ. Less potentially-leachable-nitrate was available in the root zone of RT and SP than DSP. As a result, less nitrate leached from the root zone and accumulated in the IVZ of RT and SP compared to DSP. Since soybean was not fertilized with N, the CS leached less nitrate than CC. Atrazine leaching was an insignificant problem when applied annually at 1.3 kg ha$\sp{-1}$